Many AC powered electronic devices use a microprocessor to maintain the operation of a time of day clock. Using its time base, or perhaps the 60 Hz AC frequency, the microprocessor uses internal counters to maintain the current day, hour, minute and second. As a result of this counter activity, the microprocessor must have a supply of power in order to maintain the clock.
When a traditional AC powered electronic device encounters an AC power interruption, the clock will not continue to function because the microprocessor is no longer receiving the power it needs to execute the time of day clock operation. This is unsatisfactory because it is important for many electronic devices to maintain the current time during a power interruption. In an executing mode (i.e., in order to maintain the time of day clock), the microprocessor draws power. If the power draw is excessive, batteries are relied upon to maintain the operation of the clock during the power interruption. This results in a serious drawback in consumer electronic devices because consumers cannot be relied upon to install backup batteries and even then, they do not usually replace the batteries before the energy reserve is completely depleted. Therefore, it is desirable to provide a simple, cost effective device to backup a time of day clock in the event of a power interruption.
One approach has been to slow down the operation of the microprocessor so as to reduce its power consumption. Since most consumer electronic devices use CMOS microprocessors whose power consumption is directly proportional to the frequency of operation, slowing down the microprocessor reduces the current draw. If the microprocessor is sufficiently slowed, the current draw can be lowered enough to allow a simple capacitor-based power supply to provide a temporary supply of power during a power interruption. Unfortunately, not all microprocessors have the capability to reduce their clock speed. Additionally, even at slow speeds, the microprocessors current draw is large enough to require large, expensive capacitors (commonly called ‘super caps’) to supply the power for the backup function.
Accordingly, backup power systems are increasingly used for applications, such as computer, security, data processing and communications equipment to avoid interruptions resulting from a primary AC power source. Various arrangements have been employed to provide a backup or standby power supply.
Furthermore, microwave ovens, ranges, video cassette recorders, and many other appliances include a digital clock which displays the time of day. Power for the clock typically is obtained from the AC power line which supplies power to other appliance components. If the AC power is lost, even for a brief instant, the clock must be manually reset. Although having to reset the clock is not necessarily difficult or time consuming, it can be a nuisance.
While the operation of many electronic devices depends on a clock, the basic operation of electronic clocks themselves is often interrupted when there is a power failure. If the consumer has not replaced a backup battery in the clock itself, the consumer will be required to reset all such clocks after a power interruption. This can be annoying.
Therefore, there is a need for a backup power supply for a clock or for an appliance incorporating a digital clock which is tolerant to short power outages so that the clock does not necessarily need to be reset manually after a brief, e.g., 20-30 seconds, power outage. There is a further need for such a device that does not add significant costs to an appliance.
While the inventor is aware of several backup power supply systems, these known arrangements have several disadvantages, including complexity, expense and unreliability. Furthermore, many of the known arrangements require modifications and/or direct wiring interconnections within the power supply circuit of the supported device.
Therefore, there is a need for an external backup power system capable of simply and effectively supplying DC (direct current) backup power to a supported device without requiring any modification of the supported device. It is important to provide such backup power supply that can be used with a various devices and that is inexpensive.